Currently, chemistry strips (also known as chemical strips) are in common use to test for the presence of certain chemicals in a sample solution. Typically, a single chemistry strip includes a plurality of test (chemistry) pads which are exposed to the sample to be tested. A set amount of time is allowed to pass, and then a change in the color or level of reflectance of the chemistry strip pads is measured. For example, chemistry strips for urinalysis are designed to operate on the basic premise that each test pad changes color depending on the concentration of the relevant analyte that reacts with that pad. A color chart is used to visually read the chemistry strips. The chart indicates the color of each test pad depending on the analyte concentration of the dosed sample. The original “strip reader” for chemistry strips was the human eye. The chemistry strip was held next to the color chart to assess which square on the color chart most closely matched the color on the pad, and the corresponding analyte concentration for the square on the color chart was manually recorded. This technique remains common today despite the human subjectivity of the process.
Optoelectronic strip readers have been developed to help automate the chemistry strip reading process and eliminate the subjectivity of the human observer. Most optoelectronic strip readers are basically reflectometers, that is, instruments that measure optical reflectance from the chemistry strip pads. This is done by illuminating the pad with a light source such as an LED, measuring the reflected light using a photodiode or other sensor, and comparing the result to known standards. An LED having a fairly narrow wavelength spectrum (e.g. typically about 40 nm) and a given peak wavelength is selected to maximize the change in reflectance signal vs. analyte concentration. Since the optimal wavelength is different for different chemistries, optoelectronic strip readers may employ several different color LEDs and use the various colors to measure reflectance from the chemistry pads. For typical optoelectronic strip readers, therefore, the differentiation between samples having different analyte concentrations is made by measuring differences in optical reflectance from the appropriate chemistry pad using the appropriate color LED to illuminate it.
The assumption made with conventional LED/photodiode based reflectometers to differentiate changes in color on a chemistry strip pad is that the observed change is primarily a change in brightness rather than a change in hue or saturation. For some chemistry strip pads this may be true, but for others it is not. Further, conventional reflectometers measure the reflectance of all test pads after a specific incubation period (i.e. an “end point” measurement. However, different pads may react at different rates. Further, higher concentrations of the tested material in the sample should be tested using a shorter incubation period, while lower concentrations of the tested material in the sample should be tested using a longer incubation period. By automating the inspection process so that a single measurement is taken only after incubation is complete for all pads, data that can be used to better determine material concentration levels is lost.